Medicine: Mitochondria

An ostrich egg may be considered a single living cell. Big, too, is a
single nerve cell which runs along an elephant's spine. The structure
of these monster cells is practically the same as a microscopic cell in
a rabbit's liver, of which some 980 are required to span one inch.

All such cells contain a small spherical body called a nucleus,
surrounded by a soft, jelly-like material called cytoplasm. Dotting the
cytoplasm are tiny granules called mitochondria, whose function in life
has been a mystery to physiologists.